There are several different techniques for producing a lightguide fiber for use in communications. One such technique comprises directing a constantly moving stream of reactants and oxygen through a glass substrate tube having a generally circular cross-section. The tube may be made of silicon dioxide, for example, and the reactant, silicon tetrachloride. The oxygen stream will also carry dopants to produce the appropriate or desired index of refraction in the finished lightguide fiber. The substrate glass is heated to a reaction temperature within a moving hot zone that traverses the outside of the tube, and the consequent reaction produces silicon dioxide and dopants fused into a continuous layer on the inner wall of the tube. The resulting tube is referred to as a preform tube.
A torch assembly for heating a glass substrate tube to facilitate deposition is described in U.S. Pat. No. 4,231,777 which issued on Nov. 4, 1980, in the names of B. Lynch and F. P. Partus. A plurality of nozzles which are disposed radially of a rotatably supported glass substrate tube open to an arcuate surface of a housing that is mounted on a carriage and that is spaced a predetermined distance from the tube to be heated. Initially, one end of the tube is supported in the headstock of a lathe and the other end is welded to an exhaust tube that is supported in the tailstock. Combustible gases are directed through the nozzles and toward the tube as it is turned rotatably about its longitudinal axis and as the torch assembly is moved therealong to produce a hot zone. A temperature profile is produced across the hot zone which moves along on the surface of the tube, and, hence inside the tube, with a peak value sufficient to accomplish the desired reaction and deposition. See F. P. Partus and M. A. Saifi "Lightguide Preform Manufacture" beginning at page 39 of the Winter 1980 issue of the Western Electric Engineer.
During a deposition mode, the torch carriage moves slowly from the headstock of the lathe where dopants are moved into the glass tube to the tailstock where gases are exhausted. At the end of each pass from headstock to tailstock, the torch carriage is returned rapidly to the headstock for the beginning of another cycle. The housing and the ends of the nozzles at least adjacent to the tube are cooled to an extent sufficient to eliminate substantially degradation such as, for example, by oxidation or reduction, of the material forming the housing and passageways.
Subsequent to the deposition mode, a collapse mode is used to collapse the prefrom tube into a rod-like member which is called a preform. It is this preform from which lightguide fiber is drawn. See D. H. Smithgall and D. L. Myers "Drawing Lightguide Fiber" beginning at page 49 of the hereinbefore identified Winter 1980 issue of the Western Electric Engineer.
There is still a need for improvement in the preforms manufactured by the above-described technique. It is not uncommon for the tube to become oval or to develop a bow or offset during deposition. The sag or offset that may develop during the deposition mode and any inherent tube ovality may be aggravated during collapse. This may occur, because during collapse when the torch carriage is moved and the tube is rotated more slowly, the temperatures of the preform tube are higher than during deposition. Non-straight preforms may not meet manufacturing specifications and be rejected or they may limit the amount of fiber which can be drawn therefrom. The bowed preforms that are drawn require constant operator monitoring and adjustment of a device which centers the preform with respect to a furnace that heats the preform during a drawing operation.
Other techniques in the preform tube manufacturing process contribute to its tendency to sag or to develop an offset. For example, during each return pass, the oxygen in the torch is vented with only the hydrogen gas being burned. At the headstock end, prior to the beginning of the next deposition pass, there is a sudden surge of oxygen gas. After a suitable delay, the carriage begins its next deposition pass. During each deposition pass, a pyrometer detects the surface temperature of the glass tube. The oxygen surge may cause the initial surface temperature of the glass to be greater than that required for suitable reaction. This is detected by the pyrometer which causes gas flow controllers to automatically reduce gas flow to the torch. Although the pyrometer and gas flow controllers react to excessive temperatures, the localized excess heat increases the probability for sag of the tube which is supported only at its ends. Should sag or offset develop, its severity may worsen as the deposition continues and as the tube is collapsed.
Any offset which is developed at the headstock end of the tube, if severe enough, causes oscillation of the gas flow controllers. During rotation, the offset causes part of the tube to be closer to the torch than the other part of the tube. As a result, the pyrometer, which is connected through a feedback loop to the gas flow controllers, detects, in rapid succession, fluctuating surface temperature values which cause the gas flow controllers to oscillate between positions which allow more or less gas to flow to the torch. This results in unacceptable deposition until the torch and pyrometer reach a portion of the tube that is substantially straight.
It will be recalled that the substrate tube is welded to the exhaust tube which is held in a chuck of the lathe tailstock. If the weld is such that either or both the substrate tube and the exhaust tube are eccentric to the longitudinal axis between the lathe chucks, sag or offset is imparted to the tube at the headstock end. This occurs because of the described delay, the localized heating and the longitudinal transfer of cranking motion to the softened glass zone adjacent to the headstock prior to the beginning of another pass.
Another problem relates to the cross-sectional configuration of the preform tube along its length. Often, the outer diameter of the substrate tube is not constant. Moreover, the cross-section of the tube may not be circular but may be oval-shaped. It is most desirable to correct these problems prior to the deposition mode. Conversely, it would also be desirable to be able to configure the tube in a predetermined manner such as, for example, to provide a gently tapered preform tube.
What is needed and what is not provided by the prior art are methods and apparatus for heating a glass tube to provide a substantially straight preform having a predetermined cross-sectional configuration along its length. The sought after methods and apparatus should be capable of straightening oval tubes and circular cross-section tubes in which the diameter varies along the tube. Also, the sought after methods and apparatus for straightening glass tubes should be such that other defects are not introduced into the tubes and should be capable of being integrated with the presently used torch.